apache 2.2 - Why apache2-mpm-prefork should be removed when install apache2-mpm-worker?

I have a packageA, which depends on apache2-mpm-prefork. packageA was installed. Now I tried to install apache2-mpm-worker, but the output is:

me@XXX:~$ sudo apt-get install apache2-mpm-worker 
Reading package lists... Done

Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
...
Use 'apt-get autoremove' to remove them.
The following packages will be REMOVED:
  apache2-mpm-prefork packageA
The following NEW packages will be installed:
  apache2-mpm-worker

Why? How to install apache2-mpm-worker without touching packageA?
Thanks